Considerations of energy-saving and reduction of pollution, in particular in an urban environment, are leading motor vehicle manufacturers to equip their models with an automatic starting/stopping system such as the system known by the term stop and go.
A typical stop and go situation is that of stoppage at a red light. When the vehicle stops at the light, the thermal engine is automatically stopped, then, when the light turns green, the motor is restarted by means of the reversible electrical machine, further to detection by the system that the driver has pressed the clutch pedal, or any other action which indicates the wish of the driver to restart his vehicle.
A three-phase alternator which can function as a starter, i.e. as an electric motor, is described by the company VALEO ELECTRONIQUE in French patent application FR2745445.
A rectifier bridge at the output of the alternator armature also acts as a bridge for control of the phases of the electric motor, with power transistors of the arms of the bridge being controlled by square signal sequences emitted by a control unit.
An “all or nothing” type full-wave control of this type is well known to persons skilled in the art, and is very simple to implement.
In the most recent reversible electrical machines, a polyphase synchronous rotary electrical machine is connected to a reversible direct current-alternating current converter, or inverter, which is supplied by the on-board battery in motor mode and by the electrical machine in generator mode.
The development of digital techniques makes it possible to control the inverter with pulse width modulation, commonly known as PWM, in order to obtain accurate control of the electrical machine in each particular operating condition.
Examples of PWM controls generated by a microprocessor executing programmes are described in the French patent application FR2895597 in the name of the company VALEO EQUIPEMENTS ELECTRIQUES MOTEURS.
However, the inventive body has been faced with the problem of limitation of the switching frequency of the inverters on electrical machines with a large number of poles per phase which rotate quickly.
In fact, the software processing which leads to the development of the duty cycles of the PWM controls requires a signal frequency of the PWM signals which is far higher than an electrical frequency corresponding to the speed of rotation of the machine (typically higher by an order of magnitude).
Because of the limitation of frequency of switching of the inverters, it is difficult to comply with these cycles if there is a large number of pairs of poles per phase (the electrical frequency is equal to the speed of rotation multiplied by the number of pairs of poles per phase).
A solution which is habitually adopted in these circumstances consists of controlling the phases in a conventional manner with full wave, i.e. by generating a square wave form at the electrical frequency of the machine.
This control has variable frequency by nature, since it is synchronous with the speed of the machine, and must be particularly accurate as far as the switching instants are concerned.
If it is wished to remain within a modern system based on a microprocessor executing programmes, and wherein the measurements, processing operations and commands are carried out in a constant cycle time, there is genuine difficulty in obtaining the required waveform whilst being accurate in terms of the switching instants.
In fact, only low temporal resolution is available for the position (since the ratio of calculation frequency to electrical frequency is low), and this does not make it possible to ensure the required precision concerning the form of the control voltage.